ASSESSING COASTAL FLOOD RISKS

Roger Maddrell, Chris Fleming, Chris Mounsey

Abstract

The English and Welsh coasts of the United Kingdom are protected from coastal flooding (as opposed to coastal erosion) by some 1300km of defences, containing about 2000 defence structures. Using data from the sea defence survey of these structures and over 20 years of joint probability data on sea levels and waves, Halcrow assessed the failure risks and coastal flood risks for all these defences. The risks were examined for three Bands or return periods, namely 50 years, 100 years and over 200 years. This analysis included 50 years of relative rise in sea level (isostatic and eustatic changes). The modes of defence failure examined were from overflow, overtopping and toe failure. The results were supplied to the insurance industry, as maps and on disc, as 1km map squares giving the risk band, flood depth and the postal codes in the areas. Flood risks were shown to be significant and even though reduced when more recent flood defence projects were included, they remain high. The impact of individual storms was later examined and obviously, while the risks were limited to specific areas and coasts, they were still significant. The insurance industry have used these results in order to assess their financial exposure to individual events and to their reinsurers.